1. Field of the Invention
This invention relates to a fixing device which is used in an image forming apparatus, such as a copier, a printer or the like.
2. Description of the Related Art
FIG. 8 is a diagram illustrating the schematic configuration of a conventional heating-roller-type fixing device. In FIG. 8, there is shown a heating roller 1 consisting of a releasing layer 2, a core bar 3, abd a heater 4. Also shown is a pressing roller 5 consisting of a releasing layer 6, an elastic layer 7, a core bar 8, a bearing 9, and a spring 10. Also in FIG. 8 are a thermistor 11, serving as temperature detection means, a thermoswitch 12, an entrance guide 13, a recording material 14, toner paticles 15, a separation pawl 16 and a guide 17.
The heating roller 1 comprises the core bar 3 (made of aluminum in the present embodiment) and the releasing layer 2 (in the present example, a tube made of PFA, which is a copolymer of tetrafluoroethylene and perfluoroalkylvinylether, having a thickness of 30 .mu.m is used) formed thereon. The heating roller 1 is heated by the heater 4 provided therein. The surface temperature of the heating roller 1 is detected by the thermistor 11 provided in contact with the surface of the heating roller 1. A driving circuit (not shown) for the heater 4 is controlled so as to maintain the surface temperature of the heating roller 1 at a preset temperature.
The pressing roller 5 comprises the core bar 8 (made of SUS in the present example), the elastic layer 7 (in the present example, a silicone sponge rubber, in which a conductivity-providing agent, such as carbon or the like, is dispersed) formed thereon, and the releasing layer 6 (in the present example, a PFA tube having a thickness of 50 .mu.m is used) formed on the elastic layer 7. The pressing roller 5 is pressed against the heating roller 1 by the spring 10 to form a nip portion.
An unfixed toner image formed on the recording material 14 is heated while being grasped and conveyed by the heating roller 1 and the pressing roller 5, and is fixed on the recording material 14 as a permanent fixed image by heat and pressure applied at the nip portion.
The heating roller 1 is rotatably driven by a motor (not shown).
The pressing roller 5 is not directly driven, but is driven by the rotation of the heating roller 1.
The recording material 14, on which the toner image has been fixed by the heat and pressure applied at the nip portion, is separated from the heating roller 1 by the separation pawl 16, and is discharged after passing through a sheet-discharging portion (not shown).
In the above-described conventional approach, however, the hardness of the pressing roller is in some cases reduced after extended use of the pressing roller, thereby causing the following problems.
(1) The position where the force of the spring urging the pressing roller toward the heating roller by the spring is balanced with the deformation restoring force of the elastic layer of the pressing roller moves closer to the heating roller as the hardness of the pressing roller is reduced. As a result, members disposed in the vicinity of the pressing roller (such as the entrance guide and the like) interfere with the pressing roller.
(2) Since the width of the nip portion increases, the position of the end portion of the nip portion changes. The entrance position of the recording material thereby shifts from an initial position, causing instability in the conveyance of the recording material.
(3) Since the width of the nip portion increases, creases are more easily produced in the recording material or an envelope being passed, and the obtained image will be more easily disturbed.
(4) Since the width of the nip portion increases, the recording material more easily deforms along the heating roller, thereby causing an increase in the amount of curl of the recording material. In addition, a failure in separation after fixing may occur, and a jam, folding and the like of the recording material at the sheet-discharging portion will more easily occur.
(5) Since the width of the nip portion increases, the amount of deformation of the pressing roller increases. Hence, a stress is applied to the pressing roller when it rotates, whereby the tube of the releasing layer will more easily peel off, and the rubber of the elastic layer will more easily deteriorate or rupture. Moreover, since the amount of deformation is large, the level of the compression set of the pressing roller increases.
(6) Since the width of the nip portion increases, the surface pressure at the nip portion is reduced. Hence, the frictional force with the heating roller or the recording material is reduced, causing instability in the rotation of the pressing roller. When the rotation moment to rotate the pressing roller by the frictional force is smaller than the frictional force between the core bar and the bearing and the force necessary to deform the pressing roller, the pressing roller does not rotate and slips.